use self.target_units for custom functions

[cvanoli]

The _select_buildings is a function inside the Developer object. The function selects buildings using the self.target_units, a developer property, which can be passed as a Float or as a pd.DataFrame with the column target_units. With self.target_units, a float variable called target_units is created being the self.target_units itself, it this is a Float, or a sum of the target_units column if self.target_units is a DataFrame. Then, the selection of buildings can be done through three methods: a custom selection passed by the user called custom_selection_func, the weighted_random_choice_multiparcel method from proposal_select.py or the weighted_random_choice method also from proposal_select.py. The three functions uses the target_units as inputs. The two latter, defined in proposal_select.py needs a Float type as the target_units input, so the variable target_units that was created is passed into these. But, as the custom_selection_func is a function that is defined and passed by the user, we assume that it will designed to take as input the original target_units that is passed to create the developer object. Therefore, the parameter to pass into the custom_selection_func should be self.target_units instead of target_units, since in the case self.target_units is a pd.DataFrame, the custom_selection_func will be expecting that type, instead of a Float which is the local variable target_units.

[coveralls]

Pull Request Test Coverage Report for Build 54

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.175%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
developer/develop.py	0	1	0.0%
<!--	Total:	0	1	0.0%	-->

Totals
Change from base Build 52:	0.0%
Covered Lines:	789
Relevant Lines:	829

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💛 - Coveralls

[smmaurer]

Linking things up, this fixes the bug identified in https://github.com/UDST/developer/pull/71#issuecomment-574852680.